1. Field of the Invention
This invention relates unit transition counters, and more particularly error detectors for such counters.
2. Description of the Prior Art
Fine positioning along X-Y coordinates of a movable stage is required in aerial photoplates, precision machining and many other applications. Binary code plates along the X and Y axis of the stage provided position information to X and Y encoders for detection and subsequent display. The number displayed does not always accurately reflect the actual stage position. Misalignment between the code plate and the stage axis is a frequent source of error. Electronic failure and even temporary conditions such as gliches can cause spurious data to be displayed. Further, the code plate is subject to internal defects and foreign matter deposition which introduces inconsistencies between the displayed position and the actual position. These errors are primarily noncumulative and can only be detected by slowly scanning the encoder from one position to the next and observing the numerical sequence of display position. An encoder stage 100 millimeters by 100 millimeters with one micron resolution required several hours to individually check 10,000 steps along each axis. This prior art continuity check was burdensome and frequently omitted.